1. Guide to Meteorological Instruments and Methods of Observation WMO-No. 8

2. Guide to Meteorological Instruments and Methods of Observation WMO-No. 8 Seventh edition 2008

3. WMO-No. 8 World Meteorological Organization, 2008 The right of publication in print, electronic and any other form and in any language is reserved by WMO. Short extracts from WMO publications may be reproduced without authorization, provided that the complete source is clearly indicated. Editorial correspondence and requests to publish, reproduce or translate this publication in part or in whole should be addressed to: Chairperson, Publications Board World Meteorological Organization (WMO) 7 bis, avenue de la Paix Tel.: +41 (0) 22 730 84 03 P.O. Box No. 2300 Fax: +41 (0) 22 730 80 40 CH-1211 Geneva 2, Switzerland E-mail: publications@wmo.int ISBN 978-92-63-10008-5 NOTE The designations employed in WMO publications and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of WMO concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. Opinions expressed in WMO publications are those of the authors and do not necessarily reflect those of WMO. The mention of specific companies or products does not imply that they are endorsed or recommended by WMO in preference to others of a similar nature which are not mentioned or advertised.

4. PREFACE One of the purposes of the World Meteorological Organization (WMO) is to coordinate the activities of its 188 Members in the generation of data and information on weather, climate and water, accord-ing to internationally agreed standards. With this in mind, each session of the World Meteorological Congress adopts Technical Regulations which lay down the meteorological practices and procedures to be followed by WMO Members. These Technical Regulations are supplemented by a number of Manuals and Guides which describe in more detail the practices, procedures and specifi cations that Members are requested to follow and implement. While Manuals contain mandatory practices, Guides such as this one contain recommended practices. The first edition of the Guide to Meteorological Instruments and Methods of Observation was published in 1954 and consisted of twelve chapters. Since then, standardization has remained a key concern of the Commission for Instruments and Methods of Observation (CIMO) activities, and CIMO has peri-odically reviewed the contents of the Guide, making recommendations for additions and amendments whenever appropriate. The present, seventh, edition is a fully revised version which includes additional topics and chapters refl ecting recent technological develop-ments. Its purpose, as with the previous editions, is to give comprehensive and up-to-date guidance on the most effective practices for carrying out meteorological observations and measurements. This edition was prepared through the collabora-tive efforts of 42 experts from 17 countries and was adopted by the fourteenth session of CIMO (Geneva, December 2006). The Guide describes most instruments, systems and techniques in regular use, from the simplest to the most complex and sophisticated, but does not attempt to deal with methods and instruments used only for research or experimentally. Furthermore, the Guide is not intended to be a detailed instruction manual for use by observers and technicians, but rather, it is intended to provide the basis for the preparation of manuals by National Meteorological and Hydrological Services (NMHSs) or other interested users operating observing systems, to meet their specifi c needs. However, no attempt is made to specify the fully detailed design of instruments, since to do so might hinder their further development. It was instead considered preferable to restrict standardization to the essential requirements and to confi ne recommendations to those features which are generally most common to various confi gurations of a given instrument or measurement system. Although the Guide is written primarily for NMHSs, many other organizations and research and educa-tional institutions taking meteorological observations have found it useful, so their require-ments have been kept in mind in the preparation of the Guide. Additionally, many instrument manu-facturers have recognized the usefulness of the Guide in the development and production of instru-ments and systems especially suited to Members needs. Because of the considerable demand for this publication, a decision was taken to make it availa-ble on the WMO website to all interested users. Therefore, on behalf of WMO, I wish to express my gratitude to all those NMHSs, technical commis-sions, expert teams and individuals who have contributed to this publication. (M. Jarraud) Secretary-General

5. Contents Part I. Measurement of meteorological VARIABLES Page CHAPTER 1. General......................................................................................................................... I.11 CHAPTER 2. Measurement of temperature...................................................................................... I.21 CHAPTER 3. Measurement of atmospheric pressure........................................................................ I.31 CHAPTER 4. Measurement of humidity........................................................................................... I.41 CHAPTER 5. Measurement of surface wind...................................................................................... I.51 CHAPTER 6. Measurement of precipitation..................................................................................... I.61 CHAPTER 7. Measurement of radiation........................................................................................... I.71 CHAPTER 8. Measurement of sunshine duration............................................................................ I.81 CHAPTER 9. Measurement of visibility............................................................................................ I.91 CHAPTER 10. Measurement of evaporation....................................................................................... I.101 CHAPTER 11. Measurement of soil moisture..................................................................................... I.111 CHAPTER 12. Measurement of upper-air pressure, temperature and humidity................................ I.121 CHAPTER 13. Measurement of upper wind....................................................................................... I.131 CHAPTER 14. Present and past weather; state of the ground............................................................ I.141 CHAPTER 15. Observation of clouds.................................................................................................. I.151 CHAPTER 16. Measurement of ozone................................................................................................ I.161 CHAPTER 17. Measurement of atmospheric composition................................................................. I.171

6. Guide to Meteorological Instruments and Methods of Observation Part II. Observing Systems Page CHAPTER 1. Measurements at automatic weather stations............................................................. II.11 CHAPTER 2. Measurements and observations at aeronautical meteorological stations.................. II.21 CHAPTER 3. Aircraft observations.................................................................................................... II.31 CHAPTER 4. Marine observations.................................................................................................... II.41 CHAPTER 5. Special profiling techniques for the boundary layer and the troposphere................. II.51 CHAPTER 6. Rocket measurements in the stratosphere and mesosphere........................................ II.61 CHAPTER 7. Locating the sources of atmospherics.......................................................................... II.71 CHAPTER 8. atellite observations................................................................................................... I.81 CHAPTER 9. Radar measurements.................................................................................................... II.91 CHAPTER 10. Balloon techniques...................................................................................................... II.101 CHAPTER 11. Urban observations...................................................................................................... II.111 CHAPTER 12. Road Meteorological Measurements............................................................................ II.121 Part III. QUALITY ASSURANCE AND MANAGEMENT OF OBSERVING SYSTEMS CHAPTER 1. Quality management................................................................................................... III.11 CHAPTER 2. Sampling meteorological variables.............................................................................. III.21 CHAPTER 3. Data reduction............................................................................................................. III.31 CHAPTER 4. Testing, calibration and intercomparison................................................................... III.41 CHAPTER 5. Taining of instrument specialists................................................................................. III.51 LIST OF CONTRIBUTORS TO THE GUIDE............................................................... III.31

7. Part I Measurement of meteorological VARIABLES

8. Part I. Measurement of meteorological VARIABLES contents Page CHAPTER 1. General....................................................................................................................... I.11 1.1 Meteorological observations................................................................................................. I.11 1.2 Meteorological observing systems........................................................................................ I.12 1.3 General requirements of a meteorological station............................................................... I.12 1.4 General requirements of instruments.................................................................................. I.16 1.5 Measurement standards and definitions.............................................................................. I.17 1.6 Uncertainty of measurements.............................................................................................. I.19 Annex 1.A. Regional centres .......................................................................................................... I.117 Annex 1.B. Operational measurement uncertainty requirements and instrument performance.................................................................................................................................... I.119 Annex 1.C. Station exposure description....................................................................................... I.125 References and further reading....................................................................................................... I.127 CHAPTER 2. Measurement of temperature............................................................................. I.21 2.1 General.................................................................................................................................. I.21 2.2 Liquid-in-glass thermometers............................................................................................... I.24 2.3 Mechanical thermographs.................................................................................................... I.210 2.4 Electrical thermometers........................................................................................................ I.211 2.5 Radiation shields................................................................................................................... I.216 Annex. Defining the fixed points of the international temperature scale of 1990........................ I.218 References and further reading....................................................................................................... I.220 CHAPTER 3. measurement of atmospheric pressure............................................................ I.31 3.1 General.................................................................................................................................. I.31 3.2 Mercury barometers.............................................................................................................. I.33 3.3 Electronic barometers........................................................................................................... I.38 3.4 Aneroid barometers.............................................................................................................. I.311 3.5 Barographs............................................................................................................................ I.312 3.6 Bourdon-tube barometers..................................................................................................... I.313 3.7 Barometric change................................................................................................................ I.313 3.8 General exposure requirements............................................................................................ I.314 3.9 Barometer exposure.............................................................................................................. I.314 3.10 Comparison, calibration and maintenance.......................................................................... I.315 3.11 Adjustment of barometer readings to other levels............................................................... I.320 3.12 Pressure tendency and pressure tendency characteristic.................................................... I.321 Annex 3.A. Correction of barometer readings to standard conditions.......................................... I.322 Annex 3.B. Regional standard barometers...................................................................................... I.325 References and further reading....................................................................................................... I.326 CHAPTER 4. measurement of humidity.................................................................................... I.41 4.1 General.................................................................................................................................. I.41 4.2 The psychrometer................................................................................................................. I.46 4.3 The hair hygrometer............................................................................................................. I.412 4.4 The chilled-mirror dewpoint hygrometer............................................................................ I.414 4.5 The lithium chloride heated condensation hygrometer (dew cell)..................................... I.417

9. Page I.2 Part I. Measurement of meteorological VARIABELS 4.6 Electrical resistive and capacitive hygrometers.................................................................... I.420 4.7 Hygrometers using absorption of electromagnetic radiation............................................... I.421 4.8 Safety..................................................................................................................................... I.421 4.9 Standard instruments and calibration.................................................................................. I.423 Annex 4.A. Definitions and specifications of water vapour in the atmosphere............................ I.426 Annex 4.B. Formulae for the computation of measures of humidity............................................ I.429 References and further reading....................................................................................................... I.430 CHAPTER 5. measurement of surface wind............................................................................ I.51 5.1 General.................................................................................................................................. I.51 5.2 Estimation of wind............................................................................................................... I.53 5.3 Simple instrumental methods.............................................................................................. I.54 5.4 Cup and propeller sensors.................................................................................................... I.54 5.5 Wind-direction vanes........................................................................................................... I.55 5.6 Other wind sensors............................................................................................................... I.55 5.7 Sensors and sensor combinations for component resolution.............................................. I.56 5.8 Data-processing methods..................................................................................................... I.56 5.9 Exposure of wind instruments.............................................................................................. I.58 5.10 Calibration and maintenance............................................................................................... I.511 Annex. The effective roughness length.......................................................................................... I.512 References and further reading....................................................................................................... I.513 CHAPTER 6. measurement of precipitation............................................................................ I.61 6.1 General.................................................................................................................................. I.61 6.2 Siting and exposure.............................................................................................................. I.63 6.3 Non-recording precipitation gauges..................................................................................... I.63 6.4 Precipitation gauge errors and corrections........................................................................... I.66 6.5 Recording precipitation gauges............................................................................................ I.68 6.6 Measurement of dew, ice accumulation and fog precipitation............................................ I.611 6.7 Measurement of snowfall and snow cover........................................................................... I.614 Annex 6.A. Precipitation intercomparison sites............................................................................. I.618 Annex 6.B. Suggested correction procedures for precipitation measurements.............................. I.619 References and further reading....................................................................................................... I.620 CHAPTER 7. measurement of radiation................................................................................... I.71 7.1 General.................................................................................................................................. I.71 7.2 Measurement of direct solar radiation................................................................................. I.75 7.3 Measurement of global and diffuse sky radiation................................................................ I.711 7.4 Measurement of total and long-wave radiation................................................................... I.719 7.5 Measurement of special radiation quantities....................................................................... I.724 7.6 Measurement of UV radiation.............................................................................................. I.725 Annex 7.A. Nomenclature of radiometric and photometric quantities......................................... I.731 Annex 7.B. Meteorological radiation quantities, symbols and definitions.................................... I.733 Annex 7.C. Specifications for world, regional and national radiation centres.............................. I.735 Annex 7.D. Useful formulae........................................................................................................... I.737 Annex 7.E. Diffuse sky radiation correction for a shading ring.................................................. I.739 References and further reading....................................................................................................... I.740

10. contents I.3 Page CHAPTER 8. measurement of sunshine duration................................................................. I.81 8.1 General.................................................................................................................................. I.81 8.2 Instruments and sensors....................................................................................................... I.83 8.3 Exposure of sunshine detectors............................................................................................ I.87 8.4 General sources of error........................................................................................................ I.87 8.5 Calibration............................................................................................................................ I.87 8.6 Maintenance......................................................................................................................... I.89 Annex. Algorithm to estimate sunshine duration from direct global irradiance measurements................................................................................................................................. I.810 References and further reading....................................................................................................... I.811 CHAPTER 9. measurement of visibility..................................................................................... I.91 9.1 General.................................................................................................................................. I.91 9.2 Visual estimation of meteorological optical range............................................................... I.95 9.3 Instrumental measurement of the meteorological optical range......................................... I.98 References and further reading....................................................................................................... I.915 CHAPTER 10. measurement of evaporation............................................................................ I.101 10.1 General.................................................................................................................................. I.101 10.2 Atmometers........................................................................................................................... I.102 10.3 Evaporation pans and tanks................................................................................................. I.103 10.4 Evapotranspirometers (lysimeters)....................................................................................... I.106 10.5 Estimation of evaporation from natural surfaces................................................................. I.107 References and further reading....................................................................................................... I.1010 CHAPTER 11. MEASUREMENT OF SOIL MOISTURE......................................................................... I.111 11.1 General.................................................................................................................................. I.111 11.2 Gravimetric direct measurement of soil water content......................................................... I.113 11.3 Soil water content: indirect methods................................................................................... I.114 11.4 Soil water potential instrumentation................................................................................... I.116 11.5 Remote sensing of soil moisture........................................................................................... I.118 11.6 Site selection and sample size............................................................................................... I.119 References and further reading....................................................................................................... I.1110 CHAPTER 12. measurement of upper-air pressure, temperature and humidity.......... I.121 12.1 General.................................................................................................................................. I.121 12.2 Radiosonde electronics......................................................................................................... I.126 12.3 Temperature sensors............................................................................................................. I.127 12.4 Pressure sensors..................................................................................................................... I.129 12.5 Relative humidity sensors..................................................................................................... I.1212 12.6 Ground station equipment................................................................................................... I.1215 12.7 Radiosonde operations......................................................................................................... I.1216 12.8 Radiosondes errors ............................................................................................................... I.1218 12.9 Comparison, calibration and maintenance.......................................................................... I.1228 12.10 Computations and reporting................................................................................................ I.1231 Annex 12.A. Accuracy requirements (standard error) for upper-air measurements for synoptic meteorology, interpreted for conventional upper-air and wind measurements............. I.1234

11. Page I.4 Part I. Measurement of meteorological VARIABELS Annex 12.B. Performance limits for upper wind and radiosonde temperature, relative humidity and geopotential height................................................................................................. I.1235 Annex 12.C. Guidelines for organizing radiosonde intercomparisons and for the establishment of test sites................................................................................................... I.1240 References and further reading....................................................................................................... I.1244 CHAPTER 13. measurement of upper wind.............................................................................. I.131 13.1 General.................................................................................................................................. I.131 13.2 Upper-wind sensors and instruments................................................................................... I.134 13.3 Measurement methods ........................................................................................................ I.1310 13.4 Exposure of ground equipment............................................................................................ I.1312 13.5 Sources of error..................................................................................................................... I.1313 13.6 Comparison, calibration and maintenance.......................................................................... I.1318 13.7 Corrections........................................................................................................................... I.1319 References and further reading....................................................................................................... I.1321 CHAPTER 14. present and past weather; state of the ground........................................ I.141 14.1 General.................................................................................................................................. I.141 14.2 Observation of present and past weather............................................................................. I.142 14.3 State of the ground............................................................................................................... I.145 14.4 Special phenomena............................................................................................................... I.145 Annex. Criteria for light, moderate and heavy precipitation intensity......................................... I.147 References and further reading....................................................................................................... I.148 CHAPTER 15. observation of clouds........................................................................................ I.151 15.1 General.................................................................................................................................. I.151 15.2 Estimation and observation of cloud amount, height and type.......................................... I.153 15.3 Instrumental measurements of cloud amount..................................................................... I.155 15.4 Measurement of cloud height using a searchlight............................................................... I.155 15.5 Measurement of cloud height using a balloon..................................................................... I.157 15.6 Rotatingbeam ceilometer..................................................................................................... I.157 15.7 Laser ceilometer.................................................................................................................... I.158 References and further reading....................................................................................................... I.1511 CHAPTER 16. measurement of ozone........................................................................................ I.161 16.1 General.................................................................................................................................. I.161 16.2 Surface ozone measurements................................................................................................ I.163 16.3 Total ozone measurements................................................................................................... I.164 16.4 Measurements of the vertical profile of ozone..................................................................... I.1611 16.5 Corrections to ozone measurements.................................................................................... I.1616 16.6 Aircraft and satellite observations........................................................................................ I.1617 Annex 16.A. Units for total and local ozone.................................................................................. I.1618 Annex 16.B. Measurement theory.................................................................................................. I.1620 References and further reading....................................................................................................... I.1622 CHAPTER 17. measurement of atmospheric composition................................................. I.171 17.1 General.................................................................................................................................. I.171 17.2 Measurement of specific variables........................................................................................ I.171 17.3 Quality assurance.................................................................................................................. I.1710 References and further reading....................................................................................................... I.1712

12. General 1.1 Meteorological observations 1.1.1 General Meteorological (and related environmental and geophysical) observations are made for a variety of reasons. They are used for the real-time preparation of weather analyses, forecasts and severe weather warnings, for the study of climate, for local weather-dependent operations (for example, local aerodrome flying operations, construction work on land and at sea), for hydrology and agricultural meteorology, and for research in meteorology and climatology. The purpose of the Guide to Meteorological Instruments and Methods of Observation is to support these activi-ties by giving advice on good practices for meteorological measurements and observations. There are many other sources of additional advice, and users should refer to the references placed at the end of each chapter for a bibliography of theory and practice relating to instruments and methods of observation. The references also contain national practices, national and international standards, and specific literature. They also include reports published by the World Meteorological Organization (WMO) for the Commission for Instruments and Methods of Observation (CIMO) on technical conferences, instrumentation, and international comparisons of instruments. Many other Manuals and Guides issued by WMO refer to particular applications of meteoro-logical observations (see especially those relating to the Global Observing System (WMO, 2003a; 1989), aeronautical meteorology (WMO, 1990), hydrology (WMO, 1994), agricultural meteorology (WMO, 1981) and climatology (WMO, 1983). Quality assurance and maintenance are of special interest for instrument measurements. Throughout this Guide many recommendations are made in order to meet the stated performance requirements. Particularly, Part III of this Guide is dedicated to quality assurance and management of observing systems. It is recognized that quality management and training of instrument specialists is of utmost importance. Therefore, on the recommendation of CIMO, several regional associations of WMO have Recommended by the Commission for Instruments and Methods of Observation at its ninth session (1985) through Recommendation 19. set up Regional Instrument Centres (RICs) to main-tain standards and provide advice. Their terms of reference and locations are given in Annex 1.A. The definitions and standards stated in this Guide (see section 1.5.1) will always conform to interna-tionally adopted standards. Basic documents to be referred to are the International Meteorological Vocabulary (WMO, 1992a) and the International Vocabulary of Basic and General Terms in Metrology (ISO, 1993a). 1.1.2 Representativeness The representativeness of an observation is the degree to which it accurately describes the value of the variable needed for a specific purpose. Therefore, it is not a fixed quality of any observa-tion, but results from joint appraisal of instrumentation, measurement interval and exposure against the requirements of some partic-ular application. For instance, synoptic observations should typically be representative of an area up to 100 km around the station, but for small-scale or local applications the considered area may have dimensions of 10 km or less. In particular, applications have their own preferred timescales and space scales for averaging, station density and resolution of phenomena small for agricultural meteorology, large for global long-range forecasting. Forecasting scales are closely related to the timescales of the phenomena; thus, shorter-range weather forecasts require more frequent observations from a denser network over a limited area in order to detect any small-scale phenomena and their quick development. Using various sources (WMO, 2003a; 2001; Orlanski, 1975), horizontal meteorological scales may be classified as follows, with a factor two uncertainty: (a) Microscale (less than 100 m) for agricultural meteorology, for example, evaporation; (b) Toposcale or local scale (1003 km), for exam-ple, air pollution, tornadoes; (c) Mesoscale (3100 km), for example, thunder-storms, sea and mountain breezes; (d) Large scale (1003 000 km), for example, fronts, various cyclones, cloud clusters; (e) Planetary scale (larger than 3 000 km), for example, long upper tropospheric waves. CHAPTER 1

13. Part I. Measurement I.12 fo eemotrolcgioal ARVIABLES Section 1.6 discusses the required and achievable uncertainties of instrument systems. The stated achievable uncertainties can be obtained with good instrument systems that are properly operated, but are not always obtained in practice. Good observ-ing practices require skill, training, equipment and support, which are not always available in sufficient degree. The measurement intervals required vary by application: minutes for aviation, hours for agri-culture, and days for climate description. Data storage arrangements are a compromise between available capacity and user needs. Good exposure, which is representative on scales from a few metres to 100 km, is difficult to achieve (see section 1.3). Errors of unrepresenta-tive exposure may be much larger than those expected from the instrument system in isola-tion. A station in a hilly or coastal location is likely to be unrepresentative on the large scale or mesoscale. However, good homogeneity of observations in time may enable users to employ data even from unrepresentative stations for climate studies. 1.1.3 Metadata The purpose of this Guide and related WMO publications is to ensure reliability of observa-tions by standardization. However, local resources and circumstances may cause deviations from the agreed standards of instrumentation and expo-sure. A typical example is that of regions with much snowfall, where the instruments are mounted higher than usual so that they can be useful in winter as well as summer. Users of meteorological observations often need to know the actual exposure, type and condition of the equipment and its operation; and perhaps the circumstances of the observations. This is now particularly significant in the study of climate, in which detailed station histories have to be exam-ined. Metadata (data about data) should be kept concerning all of the station establishment and maintenance matters described in section 1.3, and concerning changes which occur, including calibra-tion and maintenance history and the changes in terms of exposure and staff (WMO, 2003b). Metadata are especially important for elements which are particularly sensitive to exposure, such as precipita-tion, wind and temperature. One very basic form of metadata is information on the existence, availabil-ity and quality of meteorological data and of the metadata about them. 1.2 Meteorological observing systems The requirements for observational data may be met using in situ measurements or remote-sens-ing (including space-borne) systems, according to the ability of the various sensing systems to meas-ure the elements needed. WMO (2003a) describes the requirements in terms of global, regional and national scales and according to the application area. The Global Observing System, designed to meet these requirements, is composed of the surface-based subsystem and the space-based subsystem. The surface-based subsystem comprises a wide variety of types of stations according to the particular application (for example, surface synoptic station, upper-air station, climatological station, and so on). The space-based subsystem comprises a number of spacecraft with on-board sounding missions and the associated ground segment for command, control and data recep-tion. The succeeding paragraphs and chapters in this Guide deal with the surface-based system and, to a lesser extent, with the space-based subsystem. To derive certain meteorological obser-vations by automated systems, for example, present weather, a so-called multi- sensor approach is necessary, where an algorithm is applied to compute the result from the outputs of several sensors. 1.3 General requirements of a meteorological station The requirements for elements to be observed according to the type of station and observing network are detailed in WMO (2003a). In this section, the observational requirements of a typi-cal climatological station or a surface synoptic network station are considered. The following elements are observed at a station making surface observations (the chapters refer to Part I of the Guide): Present weather (Chapter 14) Past weather (Chapter 14) Wind direction and speed (Chapter 5) Cloud amount (Chapter 15) Cloud type (Chapter 15) Cloud-base height (Chapter 15) Visibility (Chapter 9) Temperature (Chapter 2) Relative humidity (Chapter 4)

14. Chapter 1. GENERAL I.13 Atmospheric pressure (Chapter 3) Precipitation (Chapter 6) Snow cover (Chapter 6) Sunshine and/ or solar radiation (Chapters 7, 8) Soil temperature (Chapter 2) Evaporation (Chapter 10) Instruments exist which can measure all of these elements, except cloud type. However, with current technology, instruments for present and past weather, cloud amount and height, and snow cover are not able to make observations of the whole range of phenomena, whereas human observers are able to do so. Some meteorological stations take upper-air meas-urements (Part I, Chapters 12 and 13), measurements of soil moisture (Part I, Chapter 11), ozone (Part I, Chapter 16) and atmospheric compo-sition (Part I, Chapter 17), and some make use of special instrument systems as described in Part II of this Guide. Details of observing methods and appropriate instrumentation are contained in the succeeding chapters of this Guide. 1.3.1 Automatic weather stations Most of the elements required for synoptic, clima-tological or aeronautical purposes can be measured by automatic instrumentation (Part II, Chapter 1). As the capabilities of automatic systems increase, the ratio of purely automatic weather stations to observer-staffed weather stations (with or without automatic instrumentation) increases steadily. The guidance in the following paragraphs regarding siting and exposure, changes of instrumentation, and inspection and maintenance apply equally to automatic weather stations and staffed weather stations. 1.3.2 Observers Meteorological observers are required for a number of reasons, as follows: (a) To make synoptic and/or climatological obser-vations to the required uncertainty and repre-sentativeness with the aid of appropriate instruments; (b) To maintain instruments, metadata docu-mentation and observing sites in good order; (c) To code and dispatch observations (in the absence of automatic coding and communi-cation systems); (d) To maintain in situ recording devices, includ-ing the changing of charts when provided; (e) To make or collate weekly and/or monthly records of climatological data where auto-matic systems are unavailable or inade-quate; (f) To provide supplementary or back-up obser-vations when automatic equipment does not make observations of all required elements, or when it is out of service; (g) To respond to public and professional enquiries. Observers should be trained and/or certified by an authorized Meteorological Service to establish their competence to make observations to the required standards. They should have the ability to interpret instructions for the use of instrumental and manual techniques that apply to their own particular observing systems. Guidance on the instrument training requirements for observers will be given in Part III, Chapter 5. 1.3.3 Siting and exposure 1.3.3.1 Site selection Meteorological observing stations are designed so that representative measurements (or observa-tions) can be taken according to the type of station involved. Thus, a station in the synoptic network should make observations to meet synoptic-scale requirements, whereas an aviation meteorological observing station should make observations that describe the conditions specific to the local (aerodrome) site. Where stations are used for several purposes, for example, aviation, synoptic and climatological purposes, the most stringent requirement will dictate the precise location of an observing site and its associated sensors. A detailed study on siting and exposure is published in WMO (1993a). As an example, the following considerations apply to the selection of site and instrument exposure requirements for a typical synoptic or climatologi-cal station in a regional or national network: (a) Outdoor instruments should be installed on a level piece of ground, preferably no smaller than 25 m x 25 m where there are many installations, but in cases where there are relatively few installations (as in Figure 1.1)

15. Part I. Measurement I.14 fo eemotrolcgioal ARVIABLES 1.5 m 60 cm the area may be considerably smaller, for example, 10 m x 7 m (the enclosure). The ground should be covered with short grass or a surface representative of the locality, and surrounded by open fencing or palings to exclude unauthorized persons. Within the enclosure, a bare patch of ground of about 2 m x 2 m is reserved for observations of the state of the ground and of soil temperature at depths of equal to or less than 20 cm (Part I, Chapter 2) (soil temperatures at depths greater than 20 cm can be measured outside this bare patch of ground). An example of the layout of such a station is given in Figure 1.1 (taken from WMO, 1989); (b) There should be no steeply sloping ground in the vicinity, and the site should not be in a hollow. If these conditions are not met, the observations may show peculiarities of entirely local significance; (c) The site should be well away from trees, buildings, walls or other obstructions. The distance of any such obstacle (including Cup-counter anemometer on slender 2 m pole 2 m fencing) from the raingauge should not be less than twice the height of the object above the rim of the gauge, and preferably four times the height; (d) The sunshine recorder, raingauge and anemometer must be exposed according to their requirements, preferably on the same site as the other instruments; (e) It should be noted that the enclosure may not be the best place from which to esti-mate the wind speed and direction; another observing point, more exposed to the wind, may be desirable; (f) Very open sites which are satisfactory for most instruments are unsuitable for rain-gauges. For such sites, the rainfall catch is reduced in conditions other than light winds and some degree of shelter is needed; (g) If in the instrument enclosure surroundings, maybe at some distance, objects like trees or buildings obstruct the horizon significantly, alternative viewpoints should be selected for observations of sunshine or radiation; Thermometer screen 1.5 m 1.5 m 1.5 m 1.5 m 1.5 m 100 cm Soil thermometer Raingauge 1 2 m Raingauge 2 Recording raingauge 3 m 1.5 m Soil thermometer Concrete slab Grass minimum thermometer Soil thermometers 20 cm 10 cm 5cm Bare-soil minimum thermometer Bare patch to be kept weeded Sunshine recorder on 2 m pillar 1.4 m 1.5 m 1.5 m 5 m 1.25 m 1 m 1 m 2 m 1 m 75 m Min. therm 30 cm North South West East Figure 1.1. Layout of an observing station in the northern hemisphere showing minimum distances between installations

16. Chapter 1. GENERAL I.15 (h) The position used for observing cloud and visibility should be as open as possible and command the widest possible view of the sky and the surrounding country; (i) At coastal stations, it is desirable that the station command a view of the open sea. However, the station should not be too near the edge of a cliff because the wind eddies created by the cliff will affect the wind and precipitation measurements; (j) Night observations of cloud and visibil-ity are best made from a site unaffected by extraneous lighting. It is obvious that some of the above considerations are somewhat contradictory and require compro-mise solutions. Detailed information appropriate to specific instruments and measurements is given in the succeeding chapters. 1.3.3.2 Coordinates of the station The position of a station referred to in the World Geodetic System 1984 (WGS-84) Earth Geodetic Model 1996 (EGM96) must be accurately known and recorded. The coordinates of a station are: (a) The latitude in degrees with a resolution of 1 in 1 000; (b) The longitude in degrees with a resolution of 1 in 1 000; (c) The height of the station above mean sea level, namely, the elevation of the station, to the nearest metre. These coordinates refer to the plot on which the observations are taken and may not be the same as those of the town, village or airfield after which the station is named. The elevation of the station is defined as the height above mean sea level of the ground on which the raingauge stands or, if there is no raingauge, the ground beneath the thermometer screen. If there is neither raingauge nor screen, it is the average level of terrain in the vicinity of the station. If the station reports pressure, the elevation to which the station pressure relates For an explanation of the WGS-84 and recording issues, see ICAO, 2002. Mean sea level (MSL) is defined in WMO, 1992a. The fixed reference level of MSL should be a well-defined geoid, like the WGS-84 Earth Geodetic Model 1996 (EGM96) [Geoid: the equipotential surface of the Earths gravity field which best fits, in a least squares sense, global MSL]. must be separately specified. It is the datum level to which barometric reports at the station refer; such barometric values being termed station pressure and understood to refer to the given level for the purpose of maintaining continuity in the pressure records (WMO, 1993b). If a station is located at an aerodrome, other elevations must be specified (see Part II, Chapter 2, and WMO, 1990). Definitions of measures of height and mean sea level are given in WMO (1992a). 1.3.4 Changes of instrumentation and homogeneity The characteristics of an observing site will gener-ally change over time, for example, through the growth of trees or erection of buildings on adja-cent plots. Sites should be chosen to minimize these effects, if possible. Documentation of the geography of the site and its exposure should be kept and regularly updated as a component of the metadata (see Annex 1.C and WMO, 2003b). It is especially important to minimize the effects of changes of instrument and/or changes in the siting of specific instruments. Although the static characteristics of new instruments might be well understood, when they are deployed operationally they can introduce apparent changes in site climatology. In order to guard against this eventuality, observations from new instruments should be compared over an extended interval (at least one year; see the Guide to Climatological Practices (WMO, 1983) before the old measurement system is taken out of service. The same applies when there has been a change of site. Where this procedure is impractical at all sites, it is essential to carry out comparisons at selected representative sites to attempt to deduce changes in measurement data which might be a result of changing technology or enforced site changes. 1.3.5 Inspection and maintenance 1.3.5.1 Inspection of stations All synoptic land stations and principal climato-logical stations should be inspected no less than once every two years. Agricultural meteorological and special stations should be inspected at inter-vals sufficiently short to ensure the maintenance of a high standard of observations and the correct functioning of instruments.

17. Part I. Measurement I.16 fo eemotrolcgioal ARVIABLES The principal objective of such inspections is to ascertain that: (a) The siting and exposure of instruments are known, acceptable and adequately docu-mented; (b) Instruments are of the approved type, in good order, and regularly verified against standards, as necessary; (c) There is uniformity in the methods of obser-vation and the procedures for calculating derived quantities from the observations; (d) The observers are competent to carry out their duties; (e) The metadata information is up to date. Further information on the standardization of instruments is given in section 1.5. 1.3.5.2 Maintenance Observing sites and instruments should be main-tained regularly so that the quality of observations does not deteriorate significantly between station inspections. Routine (preventive) maintenance schedules include regular housekeeping at observ-ing sites (for example, grass cutting and cleaning of exposed instrument surfaces) and manufacturers recommended checks on automatic instruments. Routine quality control checks carried out at the station or at a central point should be designed to detect equipment faults at the earliest possible stage. Depending on the nature of the fault and the type of station, the equipment should be replaced or repaired according to agreed priorities and times-cales. As part of the metadata, it is especially important that a log be kept of instrument faults, exposure changes, and remedial action taken where data are used for climatological purposes. Further information on station inspection and management can be found in WMO (1989). 1.4 General requirements of instruments 1.4.1 Desirable characteristics The most important requirements for meteoro-logical instruments are the following: (a) Uncertainty, according to the stated require-ment for the particular variable; (b) Reliability and stability; (c) Convenience of operation, calibration and maintenance; (d) Simplicity of design which is consistent with requirements; (e) Durability; (f) Acceptable cost of instrument, consumables and spare parts. With regard to the first two requirements, it is important that an instrument should be able to maintain a known uncertainty over a long period. This is much better than having a high initial uncer-tainty that cannot be retained for long under operating conditions. Initial calibrations of instruments will, in general, reveal departures from the ideal output, necessitat-ing corrections to observed data during normal operations. It is important that the corrections should be retained with the instruments at the observing site and that clear guidance be given to observers for their use. Simplicity, strength of construction, and conven-ience of operation and maintenance are important since most meteorological instruments are in continuous use year in, year out, and may be located far away from good repair facilities. Robust construc-tion is especially desirable for instruments that are wholly or partially exposed to the weather. Adherence to such characteristics will often reduce the overall cost of providing good observations, outweighing the initial cost. 1.4.2 Recording instruments In many of the recording instruments used in meteorology, the motion of the sensing element is magnified by levers that move a pen on a chart on a clock-driven drum. Such recorders should be as free as possible from friction, not only in the bearings, but also between the pen and paper. Some means of adjusting the pressure of the pen on the paper should be provided, but this pres-sure should be reduced to a minimum consistent with a continuous legible trace. Means should also be provided in clock-driven recorders for making time marks. In the design of recording instruments that will be used in cold climates, particular care must be taken to ensure that their performance is not adversely affected by extreme cold and moisture, and that routine procedures (time marks, and so forth) can be carried out by the observers while wearing gloves. Recording instruments should be compared frequently with instruments of the direct-reading type.

18. Chapter 1. GENERAL I.17 An increasing number of instruments make use of electronic recording in magnetic media or in semi-conductor microcircuits. Many of the same considerations given for bearings, friction and cold-weather servicing apply to the mechanical components of such instruments. 1.5 Measurement standards and definitions 1.5.1 Definitions of standards of measurement The term standard and other similar terms denote the various instruments, methods and scales used to establish the uncertainty of meas-urements. A nomenclature for standards of measurement is given in the International Vocabulary of Basic and General Terms in Metrology, which was prepared simultaneously by the International Bureau of Weights and Measures, the International Electrotechnical Commission, the International Federation of Clinical Chemistry, the International Organization for Standardization, the International Union of Pure and Applied Chemistry, the International Union of Pure and Applied Physics and the International Organization of Legal Metrology and issued by ISO (1993a). Some of the definitions are as follows: (Measurement) standard: A material measure, meas-uring instrument, reference material or measuring system intended to define, realize, conserve or reproduce a unit or one or more values of a quan-tity to serve as a reference. Examples: 1 kg mass standard 100 standard resistor Notes: 1. A set of similar material measures or measuring instruments that, through their combined use, constitutes a standard is called a collective standard. 2. A set of standards of chosen values that, individually or in combination, provides a series of values of quantities of the same kind is called a group standard. International standard: A standard recognized by an international agreement to serve internationally as the basis for assigning values to other standards of the quantity concerned. National standard: A standard recognized by a national decision to serve, in a country, as the basis for assigning values to other standards of the same quantity. Primary standard: A standard that is designated or widely acknowledged as having the highest metro-logical qualities and whose value is accepted without reference to other standards of the same quantity. Secondary standard: A standard whose value is assigned by comparison with a primary standard of the same quantity. Reference standard: A standard, generally having the highest metrological quality available at a given location or in a given organization, from which the measurements taken there are derived. Working standard: A standard that is used routinely to calibrate or check material measures, measuring instruments or reference materials. Notes: 1. A working standard is usually calibrated against a reference standard. 2. A working standard used routinely to ensure that measure-ments are being carried out correctly is called a check standard. Transfer standard: A standard used as an intermedi-ary to compare standards. Note: The term transfer device should be used when the intermediary is not a standard. Travelling standard: A standard, sometimes of special construction, intended for transport between differ-ent locations. Collective standard: A set of similar material measures or measuring instruments fulfilling, by their combined use, the role of a standard. Example: The World Radiometric Reference Notes: 1. A collective standard is usually intended to provide a single value of a quantity. 2. The value provided by a collective standard is an appropriate mean of the values provided by the individual instruments. Traceability: A property of the result of a measure-ment or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.

19. Part I. Measurement I.18 fo eemotrolcgioal ARVIABLES Calibration: The set of operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument or measuring system, or values represented by a material measure, and the corresponding known values of a measurand (the physical quantity being measured). Notes: 1. The result of a calibration permits the estimation of errors of indication of the measuring instrument, measuring system or material measure, or the assignment of marks on arbitrary scales. 2. A calibration may also determine other metrological properties. 3. The result of a calibration may be recorded in a document, sometimes called a calibration certificate or calibration report. 4. The result of a calibration is sometimes expressed as a calibra-tion factor, or as a series of calibration factors in the form of a calibration curve. 1.5.2 Procedures for standardization In order to control effectively the standardization of meteorological instruments on a national and international scale, a system of national and regional standards has been adopted by WMO. The locations of the regional standards for pressure and radiation are given in Part I, Chapter 3 (Annex 3.B), and Part I, Chapter 7 (Annex 7.C), respectively. In general, regional standards are designated by the regional associations, and national standards by the individual Members. Unless otherwise specified, instruments designated as regional and national standards should be compared by means of travel-ling standards at least once every five years. It is not essential for the instruments used as travelling standards to possess the uncertainty of primary or secondary standards; they should, however, be sufficiently robust to withstand transportation without changing their calibration. Similarly, the instruments in operational use at a Service should be periodically compared directly or indirectly with the national standards. Comparisons of instruments within a Service should, as far as possible, be made at the time when the instruments are issued to a station and subsequently during each regular inspection of the station, as recommended in section 1.3.5. Portable standard instruments used by inspectors should be checked against the stand-ard instruments of the Service before and after each tour of inspection. Comparisons should be carried out between oper-ational instruments of different designs (or principles of operation) to ensure homogeneity of measurements over space and time (see section 1.3.4). 1.5.3 Symbols, units and constants 1.5.3.1 Symbols and units Instrument measurements produce numerical values. The purpose of these measurements is to obtain physical or meteorological quantities representing the state of the local atmosphere. For meteorological practices, instrument readings represent variables, such as atmospheric pressure, air temperature or wind speed. A variable with symbol a is usually represented in the form a = {a}[a], where {a} stands for the numerical value and [a] stands for the symbol for the unit. General pr inciples concerning quantities, units and symbols are stated by ISO (1993b) and IUPAP (1987). The International System of Units (SI) should be used as the system of units for the evaluation of meteorological elements included in reports for international exchange. This system is published and updated by BIPM (1998). Guides for the use of SI are issued by NIST (1995) and ISO (1993b). Variables not defined as an international symbol by the International System of Quantities (ISQ), but commonly used in meteorology can be found in the International Meteorological Tables (WMO, 1966) and relevant chapters in this Guide. The following units should be used for meteoro-logical observations: (a) Atmospheric pressure, p, in hectopascals (hPa); (b) Temperature, t, in degrees Celsius (C) or T in kelvin (K); Note: The Celsius and kelvin temperature scales should conform to the actual definition of the International Temperature Scale (for 2004: ITS-90, see BIPM, 1990). (c) Wind speed, in both surface and upper-air observations, in metres per second (m s1); (d) Wind direction in degrees clockwise from north or on the scale 036, where 36 is the wind from the north and 09 the wind from the east (); (e) Relative humidity, U, in per cent (%); (f) Precipitation (total amount) in millimetres (mm) or kilograms per m2 (kg m2); The unit pascal is the principal SI derived unit for the pres-sure quantity. The unit and symbol bar is a unit outside the SI system; in every document where it is used, this unit (bar) should be defined in relation to the SI. Its continued use is not encouraged. By definition, 1 mbar (millibar) 1 hPa (hectopascal). Assuming that 1 mm equals 1 kg m2 independent of temperature.

20. Chapter 1. GENERAL I.19 (g) Precipitation intensity, Ri, in millimetres per hour (mm h1) or kilograms per m2 per second (kg m2 s1); (h) Snow water equivalent in kilograms per m2 (kg m2); (i) Evaporation in millimetres (mm); (j) Visibility in metres (m); (k) Irradiance in watts per m2 and radiant expo-sure in joules per m2 (W m2, J m2); (l) Duration of sunshine in hours (h); (m) Cloud height in metres (m); (n) Cloud amount in oktas; (o) Geopotential, used in upper-air observa-tions, in standard geopotential metres (m). Note: Height, level or altitude are presented with respect to a well-defined reference. Typical references are Mean Sea Level (MSL), station altitude or the 1013.2 hPa plane. The standard geopotential metre is defined as 0.980 665 of the dynamic metre; for levels in the troposphere, the geopotential is close in numerical value to the height expressed in metres. 1.5.3.2 Constants The following constants have been adopted for meteorological use: (a) Absolute temperature of the normal ice point T0 = 273.15 K (t = 0.00C); (b) Absolute temperature of the triple point of water T = 273.16 K (t = 0.01C), by definition of ITS-90; (c) Standard normal gravity (gn) = 9.806 65 m s2; (d) Density of mercury at 0C = 1.359 51 104 kg m3. The values of other constants are given in WMO (1973; 1988). 1.6 Uncertainty of measurements 1.6.1 Meteorological measurements 1.6.1.1 General This section deals with definitions that are relevant to the assessment of accuracy and the measurement of uncertainties in physical measurements, and concludes with statements of required and achieva-ble uncertainties in meteorology. First, it discusses some issues that arise particularly in meteorological measurements. Recommendation 3 (CBS-XII), Annex 1, adopted through Resolution 4 (EC-LIII). The term measurement is carefully defined in section 1.6.2, but in most of this Guide it is used less strictly to mean the process of measurement or its result, which may also be called an observa-tion. A sample is a single measurement, typically one of a series of spot or instantaneous readings of a sensor system, from which an average or smoothed value is derived to make an observation. For a more theoretical approach to this discussion, see Part III, Chapters 2 and 3. The terms accuracy, error and uncertainty are care-fully defined in section 1.6.2, which explains that accuracy is a qualitative term, the numerical expres-sion of which is uncertainty. This is good practice and is the form followed in this Guide. Formerly, the common and less precise use of accuracy was as in an accuracy of x, which should read an uncertainty of x. 1.6.1.2 Sources and estimates of error The sources of error in the various meteorological measurements are discussed in specific detail in the following chapters of this Guide, but in general they may be seen as accumulating through the chain of traceability and the measurement conditions. It is convenient to take air temperature as an example to discuss how errors arise, but it is not difficult to adapt the following argument to pres-sure, wind and other meteorological quantities. For temperature, the sources of error in an indi-vidual measurement are as follows: (a) Errors in the international, national and work-ing standards, and in the comparisons made between them. These may be assumed to be negligible for meteorological applications; (b) Errors in the comparisons made between the working, travelling and/or check stand-ards and the field instruments in the labora-tory or in liquid baths in the field (if that is how the traceability is established). These are small if the practice is good (say 0.1 K uncertainty at the 95 per cent confidence level, including the errors in (a) above), but may quite easily be larger, depending on the skill of the operator and the quality of the equipment; (c) Non-linearity, drift, repeatability and repro-ducibility in the field thermometer and its transducer (depending on the type of ther-mometer element); (d) The effectiveness of the heat transfer between the thermometer element and the air in the thermometer shelter, which should ensure that the element is at thermal equilibrium

21. Part I. Measurement I.110 fo eemotrolcgioal ARVIABLES with the air (related to system time-constant or lag coefficient). In a well-designed aspi-rated shelter this error will be very small, but it may be large otherwise; (e) The effectiveness of the thermometer shel-ter, which should ensure that the air in the shelter is at the same temperature as the air immediately surrounding it. In a well-designed case this error is small, but the difference between an effective and an inef-fective shelter may be 3C or more in some circumstances; (f) The exposure, which should ensure that the shelter is at a temperature which is repre-sentative of the region to be monitored. Nearby sources and heat sinks (buildings, other unrepresentative surfaces below and around the shelter) and topography (hills, land-water boundaries) may introduce large errors. The station metadata should contain a good and regularly updated description of exposure (see Annex 1.C) to inform data users about possible exposure errors. Systematic and random errors both arise at all the above-mentioned stages. The effects of the error sources (d) to (f) can be kept small if operations are very careful and if convenient terrain for siting is available; otherwise these error sources may contribute to a very large overall error. However, they are sometimes overlooked in the discussion of errors, as though the laboratory calibration of the sensor could define the total error completely. Establishing the true value is difficult in meteor-ology (Linacre, 1992). Well-designed instrument comparisons in the field may establish the char-acteristics of instruments to give a good estimate of uncertainty arising from stages (a) to (e) above. If station exposure has been documented adequately, the effects of imperfect exposure can be corrected systematically for some parameters (for example, wind; see WMO, 2002) and should be estimated for others. Comparing station data against numerically analysed fields using neighbouring stations is an effective operational quality control procedure, if there are sufficient reliable stations in the region. Differences between the individual observations at the station and the values interpolated from the analysed field are due to errors in the field as well as to the performance of the station. However, over a period, the average error at each point in the analysed field may be assumed to be zero if the surrounding stations are adequate for a sound analysis. In that case, the mean and standard deviation of the differences between the station and the analysed field may be calculated, and these may be taken as the errors in the station measurement system (including effects of expo-sure). The uncertainty in the estimate of the mean value in the long term may, thus, be made quite small (if the circumstances at the station do not change), and this is the basis of climate change studies. 1.6.2 Definitions of measurements and their errors The following terminology relating to the accuracy of measurements is taken from ISO (1993a), which contains many definitions applicable to the prac-tices of meteorological observations. ISO (1995) gives very useful and detailed practical guidance on the calculation and expression of uncertainty in measurements. Measurement: A set of operations having the objec-tive of determining the value of a quantity. Note: The operations may be performed automatically. Result of a measurement: Value attributed to a meas-urand (the physical quantity that is being measured), obtained by measurement. Notes: 1. When a result is given, it should be made clear whether it refers to the indication, the uncorrected result or the corrected result, and whether several values are averaged. 2. A complete statement of the result of a measure-ment includes information about the uncertainty of the measurement. Corrected result: The result of a measurement after correction for systematic error. Value (of a quantity): The magnitude of a particular quantity generally expressed as a unit of measure-ment multiplied by a number. Example: Length of a rod: 5.34 m. True value (of a quantity): A value consistent with the definition of a given particular quantity. = Notes: 1. This is a value that would be obtained by a perfect measurement. 2. True values are by nature indeterminate.

22. Chapter 1. GENERAL I.111 Accuracy (of measurement): The closeness of the agreement between the result of a measurement and a true value of the measurand. Notes: 1. Accuracy is a qualitative concept. 2. The term precision should not be used for accuracy. Repeatability (of results of measurements): The close-ness of the agreement between the results of successive measurements of the same measu-rand carried out under the same measurement conditions. Notes: 1. These conditions are called repeatability conditions. 2. Repeatability conditions include: (a) The same measurement procedure; (b) The same observer; (c) The same measuring instrument used under the same conditions (including weather); (d) The same location; (e) Repetition over a short period of time. 3. Repeatability may be expressed quantitatively in terms of the dispersion characteristics of the results. Reproducibility (of results of measurements): The close-ness of the agreement between the results of measurements of the same measurand carried out under changed measurement conditions. Notes: 1. A valid statement of reproducibility requires specification of the conditions changed. 2. The changed conditions may include: (a) The principle of measurement; (b) The method of measurement; (c) The observer; (d) The measuring instrument; (e) The reference standard; (f) The location; (g) The conditions of use (including weather); (h) The time. 3. Reproducibility may be expressed quantitatively in terms of the dispersion characteristics of the results. 4. Here, results are usually understood to be corrected results. Uncertainty (of measurement): A variable associated with the result of a measurement that characterizes the dispersion of the values that could be reasonably attributed to the measurand. Notes: 1. The variable may be, for example, a standard deviation (or a given multiple thereof), or the half-width of an inter-val having a stated level of confidence. 2. Uncertainty of measurement comprises, in general, many components. Some of these components may be evalu-ated from the statistical distribution of the results of a series of measurements and can be characterized by experimental stand-ard deviations. The other components, which can also be char-acterized by standard deviations, are evaluated from assumed probability distributions based on experience or other information. 3. It is understood that the result of the measurement is the best estimate of the value of the measurand, and that all components of uncertainty, including those arising from systematic effects, such as components associated with correc-tions and reference standards, contribute to the dispersion. Error (of measurement): The result of a measurement minus a true value of the measurand. Note: Since a true value cannot be determined, in practice a conventional true value is used. Deviation: The value minus its conventional true value. Random error: The result of a measurement minus the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions. Notes: 1. Random error is equal to error minus systematic error. 2. Because only a finite number of measurements can be taken, it is possible to determine only an estimate of random error. Systematic error: A mean that would result from an infinite number of measurements of the same meas-urand carried out under repeatability conditions minus a true value of the measurand. Notes: 1. Systematic error is equal to error minus random error. 2. Like true value, systematic error and its causes cannot be completely known. Correction: The value added algebraically to the uncorrected result of a measurement to compensate for a systematic error. 1.6.3 Characteristics of instruments Some other properties of instruments which must be understood when considering their uncertainty are taken from ISO (1993a). Sensitivity: The change in the response of a measur-ing instrument divided by the corresponding change in the stimulus. Note: Sensitivity may depend on the value of the stimulus.

23. Part I. Measurement I.112 fo eemotrolcgioal ARVIABLES Discrimination: The ability of a measuring instru-ment to respond to small changes in the value of the stimulus. Resolution: A quantitative expression of the ability of an indicating device to distinguish meaningfully between closely adjacent values of the quantity indicated. Hysteresis: The property of a measuring instrument whereby its response to a given stimulus depends on the sequence of preceding stimuli. Stability (of an instrument): The ability of an instru-ment to maintain its metrological characteristics constant with time. Drift: The slow variation with time of a metrologi-cal characteristic of a measuring instrument. Response time: The time interval between the instant when a stimulus is subjected to a specified abrupt change and the instant when the response reaches and remains within specified limits around its final steady value. The following other definitions are used frequently in meteorology: Statements of response time: The time for 90 per cent of the step change is often given. The time for 50 per cent of the step change is sometimes referred to as the half-time. Calculation of response time: In most simple systems, the response to a step change is: Y A e t = (1 / ) (1.1) where Y is the change after elapsed time t; A is the amplitude of the step change applied; t is the elapsed time from the step change; and is a char-acteristic variable of the system having the dimension of time. The variable is referred to as the time-constant or the lag coefficient. It is the time taken, after a step change, for the instrument to reach 1/e of the final steady reading. In other systems, the response is more compli-cated and will not be considered here (see also Part III, Chapter 2). Lag error: The error that a set of measurements may possess due to the finite response time of the observing instrument. T O Figure I.2. The distribution of data in an instrument comparison 1.6.4 The measurement uncertainties of a single instrument ISO (1995) should be used for the expression and calculation of uncertainties. It gives a detailed practi-cal account of definitions and methods of reporting, and a comprehensive description of suitable statisti-cal methods, with many illustrative examples. 1.6.4.1 The statistical distributions of observations To determine the uncertainty of any individual measurement, a statistical approach is to be consid-ered in the first place. For this purpose, the following definitions are stated (ISO, 1993; 1995): (a) Standard uncertainty; (b) Expanded uncertainty; (c) Variance, standard deviation; (d) Statistical coverage interval. If n comparisons of an operational instrument are made with the measured variable and all other significant variables held constant, if the best esti-mate of the true value is established by use of a reference standard, and if the measured variable has a Gaussian distribution, the results may be displayed as in Figure 1.2. In this figure, T is the true value, O is the mean of the n values O observed with one instrument, and is the standard deviation of the observed values with respect to their mean values. In this situation, the following characteristics can be identified: (a) The systematic error, often termed bias, given by the algebraic difference O T. Systematic errors cannot be eliminated but may often be reduced. A correction factor can be applied to compensate for the systematic effect. Typically, appropriate calibrations and However, note that several meteorological variables do not follow a Gaussian distribution. See section 1.6.4.2.3.

24. Chapter 1. GENERAL I.113 adjustments should be performed to eliminate the systematic errors of sensors. Systematic errors due to environmental or siting effects can only be reduced; (b) The random error, which arises from unpre-dictable or stochastic temporal and spatial variations. The measure of this random effect can be expressed by the standard deviation determined after n measurements, where n should be large enough. In principle, is a measure for the uncertainty of O; (c) The accuracy of measurement, which is the closeness of the agreement between the result of a measurement and a true value of the measurand. The accuracy of a measuring instrument is the ability to give responses close to a true value. Note that accuracy is a qualitative concept; (d) The uncertainty of measurement, which represents a parameter associated with the result of a measurement, that characterizes the dispersion of the values that could be reasonably attributed to the measurand. The uncertainties associated with the random and systematic effects that give rise to the error can be evaluated to express the uncertainty of measurement. 1.6.4.2 Estimating the true value In normal practice, observations are used to make an estimate of the true value. If a systematic error does not exist or has been removed from the data, the true value can be approximated by taking the mean of a very large number of carefully executed inde-pendent measurements. When fewer measurements are available, their mean has a distribution of its own and only certain limits within which the true value can be expected to lie can be indicated. In order to do this, it is necessary to choose a statistical probabil-ity (level of confidence) for the limits, and the error distribution of the means must be known. A very useful and clear explanation of this notion and related subjects is given by Natrella (1966). Further discussion is given by Eisenhart (1963). 1.6.4.2.1 Estimating the true value n large When the number of n observations is large, the distribution of the means of samples is Gaussian, even when the observational errors themselves are not. In this situation, or when the distribu-tion of the means of samples is known to be Gaussian for other reasons, the limits between which the true value of the mean can be expected to lie are obtained from: Upper limit: LU = X + k n (1.2) Lower limit: LL = X k n (1.3) where X is the average of the observations O corrected for systematic error; is the standard deviation of the whole population; and k is a factor, according to the chosen level of confidence, which can be calculated using the normal distribution function. Some values of k are as follows: Level of confidence 90% 95% 99% k 1.645 1.960 2.575 The level of confidence used in the table above is for the condition that the true value will not be outside the one particular limit (upper or lower) to be computed. When stating the level of confidence that the true value will lie between both limits, both the upper and lower outside zones have to be considered. With this in mind, it can be seen that k takes the value 1.96 for a 95 per cent probability, and that the true value of the mean lies between the limits LU and LL. 1.6.4.2.2 Estimating the true value n small When n is small, the means of samples conform to Students t distribution provided that the observational errors have a Gaussian or near- Gaussian distribution. In this situation, and for a chosen level of confidence, the upper and lower limits can be obtained from: Upper limit: LU X + t n (1.4) Lower limit: LL X t n (1.5) where t is a fa